The mechanical properties of thermoplastic articles of manufacture may be improved by stretching the thermoplastic within an optimum temperature range. For example, when producing thermoplastic fibers, it is common to stretch the fiber by drawing the fiber through a die, which reduces the diameter of the fiber and orientes the polymer molecules in the direction of drawing. This is commonly done using rollers. A first roller is temperature regulated so as to hold the temperature of the fiber at the desired temperature and a second roller is run at a speed faster than the first roller in order to stretch the fiber. The temperature at which the fiber is stretched is very important in that it determines the degree of orientation of the polymer molecules and therefore the strength and feel of the fiber.
This same method is used to stretch thermoplastic film(s) or sheet(s) or laminates formed therefrom and again it is important that the temperature be controlled in order to optimize the mechanical properties. As used herein in this specification and the claims, a laminate shall mean a laminated sheet, film, fiber, coaxial fiber, billet, etc.
Blow molding, thermoforming and the solid phase forming processes also require temperature control in order to obtain the correct melt strength and/or orientation during the forming of the article.
This is especially true during the solid phase forming processes since the thermoplastic material must be formed at temperatures just below the melt or softening point of the billet or sheet in order to obtain the optimum properties.
All of the above stretching processes typically involve the use of laminates or coaxial fibers of one or more thermoplastics. Heating the laminate or coaxial fibers and stretching them for any of the stretching processes is done while all of the layers of the thermoplastic are essentially at the same temperature.
Thus, the sheet, billet, or fiber is heated before stretching by applying sufficient thermal energy to the surface of the article to allow the article to reach an essentially even temperature profile (i.e. the temperature profile indicated by line T6 in prior art FIG. 1 where both sides are heated, or indicated by line Ts in prior art FIG. 2 where one side is heated) before stretching.
In most situations, the temperature differences between any two portions of the laminate and the time intervals between heating and stretching are such that all portions of the laminate are essentially at the same temperature during the stretching operation. In fact, it is usually the object of stretching process to get as small a temperature gradient as possible across the cross section of the sheet, film, fiber or billet during its stretching process.
It is possible to obtain a temperature gradient across the cross section of the material by heating the surface of the sheet, film, fiber or billet at a high temperature and stretching immediately. This type of heating will give a temperature profile which resembles that shown by lines T1-T5 in FIG. 1 or T0-T9 in FIG. 2. The line labelled TO in FIG. 2 describes the temperature profile at the initial stage of heating.
When one has a laminated sheet, film, fiber or billet made of two or more diverse thermoplastics whose optimum process temperatures are different, it is necessary to choose a temperature at which to process the laminate which is either best for one or another of the materials in the laminate or to choose a temperature somewhere between the optimum for each of the materials, i.e. optimum for neither. It is possible that the two materials have such diverse process temperature requirements that it is impossible to find a temperature which is appropriate for stretching. For example, when making containers, it would be desirable to use a laminate which comprises a polypropylene layer positioned adjacent an ethylene/vinyl alcohol copolymer layer. If such a laminate could be thermoformed at the optimum processing temperatures for both thermoplastics the resulting shaped laminate would exhibit excellent strength and impact properties and superior barrier properties. In most instances, however, the container maker is forced into using an ethylene/vinyl alcohol copolymer with a lower than desired vinyl alcohol content in order to get the ethylene/vinyl alcohol copolymer to process at the temperature at which they wish to process the polypropylene.
A process need be developed that allows the different layers of a laminate to be heated to their different optimum processing temperatures, prior to shaping the laminate into the desired article of manufacture. Such a process should allow each individual layer to be quickly heated to a desired narrow temperature range before the stretching process, to prevent the temperatures of adjacent layers from drifting towards one another due to conduction between the layers.